The present invention provides a method and devices for controlling the propagation of Surface Plasmon Polaritons (SPPs) using Surface Plasmon Polariton Band Gap (SPPBG) regions. The SPPBG regions are regions of one or more interfaces supporting the propagation of SPPs on which SPPs experience a periodic modulation of the dielectric properties of the media into which its electromagnetic field extend. The frequency range of the band gap is determined by the period of the modulation. SPPBG regions prohibit propagation of SPPs having a frequency within its band gap.
By forming transmitting regions in the SPPBG regions the present invention provides ultra-compact SPP waveguides. The present invention can be utilised to form compact integrated SPP/optical circuits. Also, the present invention provides cavities supporting standing SPP-waves for field localisation. Such field localisation can provide very high field intensities and can be used in various sensor applications.
The devices of the present invention provide a number of advantages over photonic components since SPPs propagates on 2-dimensional interfaces, and only confinement in the plane of propagation is needed. This allows for a very simple production of the devices according to the present invention.
Surface plasmon polaritons are quasi-two-dimensional electromagnetic (EM) modes propagating along an interface between a conducting and a dielectric material. FIG. 1 shows an SPP propagating along the interface 4 between metal 2 and air 5. The EM field amplitudes 6 decay exponentially in both neighbouring media in the directions perpendicular to the interface 4, as illustrated in FIG. 1.
Typically, SPPs are excited by matching the propagation of electromagnetic radiation from a laser beam to the propagation constant xcex2 of the SPP whereby the EM field can be coupled to SPPs. FIG. 1 shows a schematic representation of the SPP excitation in the Kretschmann configuration at a glass-metal interface 1 or an air-metal interface 4 of a metal film 2 deposited on a glass substrate 3. The angle of incidence xcex8 of the light through a glass prism 7 on the backside of the glass substrate 3 should be adjusted to satisfy the phase matching condition: xcex2=(2xcfx80/xcex) n sin xcex8, where n is the glass refractive index. The exact phase matching conditions determine which interface the plasmon is coupled to, but as the metal film 3 is typically much thinner than the transverse extension of the field amplitude 6, the SPP can be considered as primarily propagating in the dielectric layers 3 and/or 5 and following the metal-dielectric interface 2 and/or 4.
Several methods and devices for performing this coupling are known; e.g. prism couplers as illustrated in FIG. 1 or described in U.S. Pat. No. 4,565,422 and grating couplers such as described in U.S. Pat. No. 4,567,147 and U.S. Pat. No. 4,765,705. The propagating SPPs can be converted back to photons again by making use of a similar device.
Some simple optical elements able to govern SPP propagation have been suggested by Smolyaninov et al. (Phys. Rev. B 56, 1997, 1601). These elements utilise diffraction and refraction of SPPs on surface defects according to the Huygens-Fresnel principle.
The existence of surface plasmon polariton band structure have been mentioned in a number of articles such as Scherer et al. (Journal of Lightwave Technology 17, 1999, 1928); Smolyaninov et al. (Phys. Rev. B 59, 1999, 2454); and Kitson et al. (1996) (Phys. Rev. Lett. 77, 1996, 2670). Such band structures arise from periodic structures fabricated at a metal-dielectric interface. When the excited SPP propagate along the periodic structure, the SPP propagation constant is periodically modulated resulting eventually in a xe2x80x9cplasmonic band gapxe2x80x9d effect.
Plasmonic band gaps structures in 2-dimensional crystals have been reported by Kitson et al (1996). The article describes the coupling of photons to SPPs on a textured interface using a prism as described in relation to FIG. 1 of the present description, the texture describing a periodic hexagonal pattern. The reflection of the incident laser beam is a measure of the coupling of photons to SPPs on the interface and FIG. 3 of the article illustrates the resulting reflectivity of the coupling region. Thus, FIG. 3 shows that for photons having energy in the interval 1.91-2.00 eV, there is a poor coupling of photons to SPPs illustrating a plasmonic band gap for the corresponding SPPs.
A further article by Kitson et al. (J. Appl. Phys., 84, 1998, 2399) relates to reducing losses in microcavities using metallic mirrors (e.g. organic LEDs). The article proposes a method to avoid losses due to non-radiative coupling from microcavity modes to SPP modes in the metal mirrors. Using textured mirror surfaces, a band gap may be introduced, which prohibit coupling to SPPs having energies within the band gap (the prohibition of this coupling is described in detail in the article by the same authors in the previous section, Kitson et al. (1996)). Tuning the band gap to the microcavity mode will thereby reduce the coupling losses of the microcavity mode. The article describes a microcavity with a one-dimensional texturing of one of the metal mirrors.
Photonic band-gap (PBG) materials have been used for providing wave guiding, light localisation, low losses for bending and strong wavelength dependent light transmission. The photonic band gap effect relies on periodic scattering of light by a wavelength scale periodic structure of scatters similar to the effect experienced by electrons in atomic lattices, namely that the photon/electron energies are arranged into energy bands separated by gaps in which propagation states are prohibited. Existing PBG-based structures utilises 3-dimensional periodic structures which are typically difficult to fabricate and have so far very little design flexibility. Also, existing planar PBG based waveguides have high optical losses in the out of plane dimension.
It is an object of the invention to provide a method and a device for guiding and localisation of electromagnetic radiation.
It is another object of the invention to provide a substantially 2-dimensional structure for guiding of electromagnetic radiation.
It is a further object of the invention to provide compact and low loss integrated optical circuits comprising passive and active components such as waveguides, bends, splitters, couplers, filters, multiplexers, de-multiplexers, interferometers, resonators, sensors, tuneable filters, amplifiers, switches, sensors, etc.
It is a still further object of the invention to provide compact and low loss integrated optical circuits, which can process signals faster than known optical circuits due to their smaller size.
It is a still further object of the invention to provide compact and low loss integrated optical circuits, which are easy and cheap to fabricate.
It is a still further object of the invention to provide localised high intensity electromagnetic fields for use in sensor applications.
The present invention fulfil these objects by providing a method and a device providing a controlled propagation of Surface Plasmon Polaritons (SPPs) in Surface Plasmon Polariton Band Gap (SPPBG) structures. By leaving channels in SPPBG structures free from periodic modulation, the present invention provides ultra-compact waveguides in SPPBG structures, that is, an energy/frequency dependent guiding of the SPPs which can be utilised to form compact integrated SPP/optical circuits.
Thus, the present invention is based on processing light signals in a 2-dimensional system by guiding and/or localising corresponding SPP fields. The coupling of light signals to interface propagating EM fields (SPPs) can be done with a close to 100% conversion efficiency in a thin conducting film using gratings or prism couplers. The SPPs have a wavelength very similar to that of photons and the same signal processing opportunities using band gap effects can be utilised.
It is an important property of SPPs that the EM field is constrained to propagate along the conductor-dielectric interface(s), wherefore an SPP component can be considered a 2-dimensional system. However, it is an equally important property of SPPs that the EM field extends into the dielectric material above and/or below the 2-dimensional interface, wherefore the SPP propagation depends on the properties of the dielectric material (if the metal layer is thin, the fields extends into dielectric materials on both sides of the layer). Therefore the properties in relation to scattering are different for SPPs as compared to photons. A scattering centre in the interface interacting with an SPP may result in a scattered EM field propagating away from the interface as photons, or stay at the interface as an SPP. If scattering of a beam of SPPs results in coupling EM radiation to a mode, which is not directly coupled back to the beam of SPPs, it will introduce severe losses.
A device may comprise other conductor-dielectric interfaces in the vicinity of a conductor-dielectric interface holding an SPP. An SPP may couple to SPP modes at other interfaces if the interfaces lie closer together than the extension of the EM field of the SPP. If the conducting material is very thin, such as a metal film, it can itself provide two interfaces supporting SPP modes. An SPP coupled to SPP modes of one interface of a thin layer of conducting material can couple with SPP modes on the other. This combination of modes generates two combined modes propagating on, or tied to, both conductor-dielectric interfaces as shown in FIG. 1B. A symmetric mode 8 having high EM field amplitudes in the conducting layer 2 and tales into the dielectric layers 3 and 5 on both sides, and an asymmetric mode 9 having low EM field amplitudes in the conducting layer 2 and high amplitudes in the dielectric layers 3 and 5. The symmetric mode 8 is similar to the normal SPP mode 6 propagating on one interface in FIG. 1A. However, the asymmetric mode 9 represents what is called a Long-Range Surface Plasmon Polariton (LR-SPP) (See e.g. S. Glasberg, Appl. Phys. Lett. 70, 1210 (1997) and references therein). LR-SPPs are allowed to propagate with less damping due to their small amplitude in the conducting layer 2. In the terminology of the present invention, an SPP may be an SPP propagating on one interface or an LR-SPP propagating on two interfaces of a thin conducting layer.
However, as is shown by experiments performed in accordance with a preferred embodiment of the present invention, notwithstanding the losses from scattering of SPPs, it is still possible to provide a localisation of SPPs using SPPBG structures resulting in guiding over distances of more than a few wavelengths and longer, this is especially true for structures supporting LR-SPPs.
According to a first aspect, the present invention provides a device for guiding surface plasmon polaritons (SPPs) having a first frequency, said waveguide device comprising,
a first medium having a first interface to a second medium, said interface being adapted to guide surface plasmon polaritons and being at least substantially plane, and
a plurality of scattering centres, each scattering centre being a region whose cross section in a plane at least substantially parallel to the first interface is an area having a complex dielectric constant different from the complex dielectric constants of the surrounding areas in said plane,
wherein projections of said scattering centres at least substantially perpendicularly onto the first interface define one or more non-transmitting parts and one or more transmitting parts on the first interface by forming predetermined, at least substantially periodic patterns of projected scattering centres in said non-transmitting parts, thereby making the non-transmitting parts SPPBG (surface plasmon polariton band gap) regions adapted to at least substantially prohibit the propagation of SPPs having the first frequency, and by not forming the predetermined pattern in the one or more transmitting parts, and
wherein the plurality of scattering centres are positioned so as to define at least one transmitting part being at least partially surrounded by one or more non-transmitting parts on the first interface.
A surface plasmon polariton is an EM wave propagating as a charge density oscillation on an interface between two materials. In order for the EM wave to propagate on the interface, the interface must be between a first material having, at the frequency of the wave, AC conductivity and a second material which are both transparent to EM radiation at the frequency of the wave. These conditions can be expressed in the frequency dependent complex dielectric constant ∈ of the materials.
The dielectric properties of materials are strongly dependent on the frequency of the applied EM fields. The frequency ranges in which a material has a complex dielectric constant ∈ with a negative or positive real part Re(∈) depends on a number of properties such as the character of the free and bound charges in the material. Typically, for each material or material composition at a given temperature, pressure etc., there will be a frequency above which the complex dielectric constant of the material has a positive real part, Re(∈) greater than 0, and below which the complex dielectric constant of the material has a negative real part, Re(∈) less than 0. Thus, the first and second frequency ranges are typically bound only in one end.
Hence, preferably the interface is between the first medium having a first complex dielectric constant ∈1 with a negative real part, Re(∈1) less than 0, in a first frequency range, and a second medium having a second complex dielectric constant ∈2 with a positive real part, Re(∈2) greater than 0, in a second frequency range. Both the first and second frequency ranges comprise the first frequency. The two materials forming the first interface is thus preferably a conducting material and a dielectric material, the conducting material being the first medium and the dielectric material being the second medium.
The non-transmitting parts are parts in which propagating SPPs having the first frequency will experience an SPPBG at least substantially prohibiting the propagation of the SPP. The frequency range within which the SPPBG will prohibit propagation of SPPs depends on the spatial period of the at least substantially periodic pattern of projected scattering centres. The transmitting parts are parts in which SPPs having the first frequency can propagate freely. However, the transmitting parts does not necessarily support the propagation of SPPs of any frequency, since the transmitting parts may establish an SPPBG having a band gap not comprising frequencies different from the first frequency.
Thus, the transmitting parts either has a different pattern of projected scattering centres than the non-transmitting parts or is at least substantially void of periodic patterns of projected scattering centres. The transmitting parts may therefore also comprise an at least substantially periodic pattern of projected scattering centres having a spatial period different from the predetermined pattern. Hence, the transmitting parts of the first interface may comprise a pattern of projected scattering centres similar to the predetermined pattern but having deviations such as resulting from one or more abnormal, missing or displaced scattering centres so as for the transmitting parts of the first interface not to establish an SPPBG.
SPPs having a frequency within the predetermined frequency range can not propagate on the non-transmitting parts of the first interface, whereas they may propagate on the transmitting parts. Hence, SPPs having a frequency within the predetermined frequency range and propagating on a transmitting part will be at least partly reflected when incident on an interface between a transmitting and a non-transmitting part.
In order to provide an SPPBG effect, the scattering centres should form a periodic pattern as seen from a propagating SPP, thus, the pattern have an extension at least substantially parallel to the first interface of the first medium on which the SPPs propagates. This is not to be interpreted as meaning that all scattering centres must lie in substantially the same plane. Actually, scattering centres may lie at different distances from the first interface, thereby contributing differently to the all-over SPPBG effect. However, in order for a scattering centre to cause scattering of an SPP propagating on the first interface, the SPP shall have non-vanishing field amplitude at the position of the scattering centre. The penetration depth of the field amplitude of an SPP having a frequency in the visible range into air is typically less than 1000 nanometers (1 nanometer=10xe2x88x929 m=1 nm).
The majority of the scattering centres are preferably positioned within an at least substantially planar region having an extension at least substantially parallel to the first interface. Preferably, the at least substantially planar region is located within a distance of less than 1000 nm from the first interface. Optionally, the at least substantially planar region is located within a distance of less than 500 nm from the first interface, such as less than 100 nm or less than 50 or 10 nm.
The at least substantially planar region comprising at least a majority of the scattering centres, such as all the scattering centres may be positioned in different media depending of the specific material structure of the device. In a preferred embodiment, at least part of the at least substantially planar region is comprised by the first medium. In another preferred embodiment at least part of the at least substantially planar region is comprised by the second medium.
The second medium preferably comprises one or more materials selected from the group consisting of: SiO2, air, polymers, Al2O3 (sapphire), quarts, and limeglass. Also, the first medium may preferably comprise one or more materials selected from the group consisting of: Au, Cu, Ag, Al, Cr, Ti, Pt, Ni, Ge, Si, Pd, and superconductors. In a preferred embodiment, the first medium comprises a conducting thin film supported by the second medium.
Alternatively, at least part of the at least substantially planar region is comprised by a third medium adjacent to the second medium. Since the material structure of the device typically comprise various layers of deposited material, the device may further comprise one or more material layers between the second and the third medium.
In order to support LR-SPPs, the first medium is preferably a thin conducting film supported by the second medium, so that coupling between SPP modes on either side of the film is made possible. Preferably, the film has a thickness smaller than 100 nm. However, the thinner the film, the smaller the damping of the LR-SPPs in the film is. Hence, preferably the film has a thickness smaller than 50 nm, such as smaller than 25 nm, 10 nm, 5 nm, 2 nm, 1 nm, 0.5 nm, 0.25 nm, or 0.1 nm. Also, to optimise the propagation of LR-SPPs, the dielectric materials sandwiching the thin conducting film preferably has similar dielectric properties, such as identical dielectric properties.
In a preferred embodiment, the second medium comprises a gain medium for coupling energy to SPP modes supported by the first interface. The gain medium may be e.g. electrically or optically pumped.
The non-transmitting parts can be used to control the propagation of an SPP by restricting the areas on the first interface on which the SPP can propagate. In the present application, the term guiding designates any control of the propagation of an SPP by use of non-transmitting or parts or equivalent. Hence an SPP waveguide is a device for guiding SPPs by controlling the propagation of an SPP from one point on the first interface to another. An SPP waveguide is thus a transmitting part which is partly surrounded by non-transmitting parts so as to form a channel or a guide through the non-transmitting parts. Alternatively, a device for guiding SPPs may be a mirror or a grating for deflecting incident SPPs or a filter for deflecting transmitting SPPs depending on their frequency.
A transmitting part surrounded by non-transmitting parts so as to localise an EM field by supporting standing SPP-waves may form an SPP cavity. The period of the predetermined pattern of some of the non-transmitting parts surrounding a cavity may be de-tuned so as to have a small transmittivity thereby providing an output coupler from the cavity.
The device preferably further comprises one or more input coupling structures for coupling photons to SPPs in a controlled manner and or one or more output coupling structures for coupling SPPs to photons in a controlled manner.
Now, having provided a device which can guide SPP using transmitting and non-transmitting parts, it is possible to form components and circuits for processing SPPs.
Hence, in a second aspect, the present invention provides an SPP component comprising an SPP receiving part comprising an input coupling structure for receiving one or more SPPs and an SPP waveguide according to the first aspect of the present invention for deflecting at least part of one of the one or more received SPPs.
The SPP component preferably further comprises at least one active region having a controllable complex refractive index for inducing phase and/or amplitude modulations in guided SPPs, the SPP component further comprising means for controlling the complex refractive index of the active region. The active region preferably lies in the dielectric material within the extent of the EM of the SPPs. Also, the SPP waveguides of the SPP component may form an interferometer comprising the at least one active region.
In another embodiment, the SPP component preferably further comprises a further non-transmitting part being an SPPBG region adapted to at least substantially prohibit the propagation of SPPs having a frequency different from the first frequency, in order to form a wavelength filter for SPPs.
Similarly, in a third aspect, the present invention provides an SPP circuit comprising:
an input structure for coupling photons to SPPs,
at least one output structure for coupling SPPs t o photons,
one or more SPP component s according to claims 20-23, and
two or more SPPBG waveguides according to claim 2 for guiding SPPs from the input structure to one of the one or more SPP components, and for guiding SPPs from one of the one or more SPP component s to the at least one output structure.
According to a fourth aspect, the present invention provides a method for controlling the propagation of surface plasmon polaritons (SPPs) propagating on an at least substantially planar interface between a first and a second medium, said method comprising the steps
providing the first medium, the first medium comprising a first material layer having a first complex dielectric constant ∈1 with a negative real part, Re(∈1) less than 0, in a first frequency range and having a first surface abutting the interface,
providing the second medium, the second medium having, in at least some parts abutting the interface, a second complex dielectric constant ∈2 with a positive real part, Re(∈2) greater than 0, in a second frequency range at least in one or more parts abutting the interface,
propagating an SPP at the interface, said SPP having a first frequency comprised in the first and second frequency range,
defining a propagation layer comprising the interface and surrounding regions, wherein every point is subject to an electromagnetic field of the SPP having a strength not less than 1% of an electromagnetic field at the interface when the SPP propagates on the part of the interface closest to the point, and
confining the SPP to a transmitting part of the interface by providing one or more non-transmitting parts of the interface being SPPBG (surface plasmon polariton band gap) regions at least substantially inaccessible to SPPs having a frequency within a third frequency range comprising the first frequency, said SPPBG regions being defined by a plurality of scattering centres in the propagation layer forming a predetermined, at least substantially periodic pattern when projected at least substantially perpendicularly onto the first interface, each scattering centre being a region whose cross section, in a plane at least substantially parallel to the interface, is an area having one or more complex dielectric constants different from the complex dielectric constant of the surrounding areas in said plane.
SPPs having a frequency within the third frequency range can only propagate on the transmitting part. The frequency of an electromagnetic wave is typically a frequency range characterised by a frequency distribution having a given width and a centre frequency, the frequency of the wave typically refers to the centre frequency.
The SPPBG structure needed to guide the plasmon waves can be obtained by forming a 2D lattice of scattering centres. Scattering centres are typically periodic variations in the complex dielectric constant of the SPP carrying media or local geometric deformations in the interface. Since an SPP is an electromagnetic wave, scattering centres need not to be formed in, or in contact to, the interface. Periodic variations in the complex dielectric constant will provide scattering centres to an SPP if the field amplitudes is non-vanishing at the position of the variation, and hence may well lie anywhere within the electromagnetic field of the SPP.
An SPP wave is confined to move on the interface, but its field is not confined to the interface as can be seen in FIG. 1. Thus, the electromagnetic field amplitudes of an SPP extent into the neighbouring media in the directions perpendicular to the interface. The extent depends on the SPP mode and on the dielectric properties of the materials in a given direction. This means that there is a region surrounding an interface on which an SPP propagates wherein the electromagnetic field of the SPP is non-vanishing. Given that the interface is preferably planar, this region of non-vanishing fields will establish a propagation layer of varying thickness.
Any variation of the complex dielectric constant at a point with non-vanishing SPP field amplitude will give rise to some scattering. Hence, in order for a scattering centre to be seen by a propagating SPP, the scattering centre preferably resides in the propagation layer as a local variation in the complex dielectric constant in a plane at least substantially parallel to the first interface on which the SPP propagate. Using a plane to define a scattering centre allows for each centre, and its corresponding intersecting plane, to be positioned at any vertical position in the propagation layer. The plane corresponding to a scattering centre is specific to the individual scattering centre and hence not the same plane as used to describe the extension of the pattern.
As seen from a position outside the propagation layer, the predetermined pattern may be projected perpendicular onto the interface and thereby form the non-transmitting parts in which SPPs experience the predetermined pattern and the transmitting parts in which they do not. For an SPP propagating on the interface and having a frequency within the SPPBG of the predetermined pattern, the non-transmitting parts of the second interface are forbidden regions wherefore the SPP will be confined to the transmitting parts experiencing an at least substantially total reflection when propagating towards the non-transmitting parts.
However, the strength of the SPPBG effect from a pattern of scattering centres will depend of the SPP field amplitudes at the position of the scattering centre. Scattering centres experiencing high field amplitudes will give rise to strong SPPBG effects whereas scattering centres experiencing low field amplitudes will give rise to weak SPPBG effects. The strength of the SPPBG effect from a pattern of scattering centres may also depend on the contrast of the complex dielectric constant of the scattering centre to the complex dielectric constant of the surrounding media.
In order to obtain a significant contribution from a scattering centre, the scattering centres are preferably comprised within a propagation layer defined as regions wherein the field strength from an SPP are larger than 0.1% of the field strength at the interface, such as larger than 0.5% or larger than 1% or 2% of the field strength at the interface, even, to obtain stronger SPPBG effects, larger than 5% or 10% such as larger than 25% or 50% of the field strength at the interface. Optionally, the scattering centres are positioned in contact with the interface or formed as geometrical deformation of the interface.
In a preferred method according to the second aspect of the present invention, the scattering centres are structures formed at the interface in the first and/or second media.
The step of propagating the SPP preferably further comprises the step of propagating the SPP on the transmitting part of the interface. Also, the step of confining the SPP to the transmitting part of the interface preferably further comprises the step of, whenever the SPP propagates from the transmitting part of the interface into the non-transmitting part of the interface, reflecting at least part of the SPP on the non-transmitting part of the interface and propagating the reflected part of the SPP on the transmitting part of the interface.
The first and non-transmitting parts of the interface may thereby define an SPPBG waveguide or cavity, depending on the topology of the transmitting and non-transmitting parts. An SPPBG waveguide is typically formed by one or more connected transmitting parts leading from one position to another and used to transport and/or process a light signal propagating between the position. Hence, an SPPBG waveguide is typically formed by transmitting parts which are connected to photon/SPP-couplers or SPP-components. An SPPBG cavity is typically an isolated transmitting part being closed in the sense that it supports standing SPP waves. An SPPBG cavity may be formed by a single deviation such as an omitted or abnormal scattering centre. An extremely high intensity can be obtained from localised field of standing SPP-waves by continuously coupling photons to a cavity, e.g. for use in sensor applications.
In order for the transmitting part of the interface to support propagation of SPPs, the corresponding parts of the propagation layer may comprise scattering centres as long as these do not provide an SPPBG. Hence, parts of the propagation layer comprising the transmitting part of the interface may comprise a pattern of scattering centres different from the predetermined pattern or be at least substantially void of scattering centres.
The method may be applied in relation to the optical signals, thus, the method may further comprise the step of forming the SPP by coupling one or more photons to the interface.
Similarly, the method may further comprise the step of coupling at least part of the SPP to one or more photons.
In order to utilise the method in relation to signal processing wherein a plurality of electromagnetic waves are controlled. Hence, the method may further comprise the step of propagating a SPP on the interface, said second SPP having a second frequency.
The second SPP may be provided at a different time than the first SPP (time division) and/or the second frequency may be different from the first frequency (wavelength division).
If the second frequency is different from the first frequency and outside the third frequency interval, the step of confining the SPP to the transmitting part of the interface may further comprise the step of propagating the second SPP on one of the non-transmitting parts of the interface. Thereby the first and second SPP can add and/or separate on the interface. This step may be used as a wavelength filter for use in e.g. wavelength division multiplexing (WDM) of signals.
Also, the band gap of the SPPBG may be adjusted in order to control the propagation of the second SPP. Hence, the method may further comprise the steps of
changing the third frequency range so as to comprise the second frequency, and
reflecting at least part of the second SPP on the non-transmitting part of the interface whenever the second SPP propagates from the transmitting part of the interface into the non-transmitting part of the interface, and propagating the reflected part of the second SPP on the transmitting part of the interface.
In a preferred embodiment, the SPPBG is modulated so as to modulate the second SPP. The propagation region may comprise electro-optic materials whereby the complex dielectric constant of the materials forming the predetermined pattern, and thereby also the band gap, may be adjusted electrically for use in e.g. time divisional multiplexing.
In another preferred embodiment, the SPPBG may be amplified or modulated by further providing the steps of:
providing a gain medium in the second medium for coupling energy to SPP modes supported by the first interface, said gain medium defining a transmitting part of the first interface when projected perpendicularly onto the first interface,
pumping the gain medium electrically or optically, and
amplifying the guided SPP by coupling energy from the gain medium to the mode containing the SPP.
In a fifth aspect, the present invention provides a first method for manufacturing a device for controlling the propagation of an SPP (surface plasmon polariton) having a first frequency and propagating on an at least substantially planar interface, said method comprising the steps of:
providing a substrate having an at least substantially planar surface, forming, in one or more parts of the substrate surface, a predetermined and at least substantially periodic pattern of structures which are concave or convex with respect to the substrate surface so as to define one or more non-transmitting parts of an interface between the substrate and a first material layer held by the substrate surface so as to form concave or convex structures in the first layer which are associated with the concave or convex structures of the substrate surface, the first material layer having a complex dielectric constant of with a negative real part, Re(∈1) less than 0, in a first frequency range comprising the first frequency, the interface being an upper or a lower surface of the first material layer,
wherein the pattern of structures is formed so as to define one or more transmitting parts on the interface without said predetermined pattern, said transmitting parts being at least partly surrounded by one or more non-transmitting parts of the substrate surface.
Preferably, the interface is adapted to support the propagation of SPPs and the one or more non-transmitting parts of the interface provides one or more SPPBG (surface plasmon polariton band gap) regions on the interface.
In order to form SPP waveguides or cavities, the transmitting parts establish regions of the interface in which the at least partly surrounding non-transmitting parts confine the SPPs. In a preferred embodiment, the first method for manufacturing comprises the step of forming the pattern of structures so as to define one or more channels of transmitting parts in the non-transmitting parts for establishing an SPP waveguide in the non-transmitting parts on the interface. In another preferred embodiment, the first method for manufacturing comprises the step of forming the pattern of structures so as to define a transmitting part being surrounded by non-transmitting parts on the interface in order to establish an SPP cavity.
In a sixth aspect, the present invention provides a second method for manufacturing a device for controlling the propagation of an SPP (surface plasmon polariton) having a first frequency and propagating on an at least substantially planar interface, said method comprising the steps of:
providing a first material layer having a complex dielectric constant ∈1 with a negative real part, Re(∈1) less than 0, in a first frequency range comprising the first frequency, the interface being defined as the plane associated with an upper or a lower surface of the first material layer, and
removing selected regions of, or altering the complex dielectric constant of selected regions of, the first layer so as to form a predetermined, at least substantially periodic pattern of selected regions in the first layer, the selected regions having a complex dielectric constant different from ∈1,
wherein the selected regions define one or more non-transmitting parts and one or more transmitting parts of an interface between the first layer and a second medium, and wherein said transmitting parts being at least partly surrounded by the one or more non-transmitting parts of the first layer.
Preferably, the interface is adapted to support the propagation of SPPs and the one or more non-transmitting parts of the interface provides one or more SPPBG (surface plasmon polariton band gap) regions on the interface.
In order to form SPP waveguides or cavities, the transmitting parts establish regions of the interface in which the at least partly surrounding non-transmitting parts confine the SPPs. In a preferred embodiment, the first method for manufacturing comprises the step of forming the pattern of regions so as to define one or more channels of transmitting parts in the non-transmitting parts for establishing an SPP waveguide in the non-transmitting parts on the interface. In another preferred embodiment, the first method for manufacturing comprises the step of forming the pattern of regions so as to define a transmitting part being surrounded by non-transmitting parts on the interface in order to establish an SPP cavity.
In a seventh aspect, the present invention provides a third method for manufacturing a device for controlling the propagation of an SPP (surface plasmon polariton) having a first frequency and propagating on an at least substantially planar interface between a material layer and a medium, said method comprising the steps of:
providing the substrate having a surface abutting the material layer, the substrate having a complex dielectric constant ∈2 with a positive real part, Re(∈2) greater than 0, in a second frequency range comprising the first frequency, and
altering the complex dielectric constant of a plurality of regions in the substrate to a complex dielectric constant different from ∈2, said plurality of regions being positioned so as to, when projected at least substantially perpendicularly onto the interface, form one or more predetermined, at least substantially periodic patterns defining one or more non-transmitting parts of the interface.
providing the material layer on the surface of the substrate, said material layer having a first complex dielectric constant ∈1 with a negative real part, Re(∈1) less than 0, in a first frequency range comprising the first frequency and having an upper and a lower surface, the interface being defined as the plane associated with the upper or the lower surface of the material layer,
wherein the one or more predetermined patterns define one or more transmitting parts of the interface without the predetermined pattern, said transmitting parts being at least partly surrounded by the one or more non-transmitting parts.
Preferably, the interface is adapted to support the propagation of SPPs and the one or more non-transmitting parts of the interface provides one or more SPPBG (surface plasmon polariton band gap) regions on the interface.
Preferably, the medium forming the interface with the material layer is the substrate. The substrate may however comprise one or more layers of different material composition.
In order to form SPP waveguides or cavities, the transmitting parts establish regions of the interface in which the at least partly surrounding non-transmitting parts confine the SPPs. In a preferred embodiment, the first method for manufacturing comprises the step of positioning the plurality of regions so as to define one or more channels of transmitting parts in the non-transmitting parts for establishing an SPP waveguide in the non-transmitting parts on the interface. In another preferred embodiment, the first method for manufacturing comprises the step of positioning the plurality of regions so as to define a transmitting part being surrounded by non-transmitting parts on the interface in order to establish an SPP cavity.
It is a common feature for all the predetermined patterns forming SPPBG regions according to the present invention, that it is the spatial period and the structure (meaning the 2-dimensional lattice structure as defined by a unity cell) of the predetermined pattern which determines the predetermined frequency range of the band gap. The frequencies comprised in the predetermined frequency range of an SPPBG are the frequencies of the guided SPPs in the material configuration of the device. Given a specific device, it is often possible to find a relationship between the spatial period xcex9 and the wavelengths xcexSPP of SPPs having a frequency in the predetermined frequency range. Preferably the spatial period of the predetermined pattern is within the intervals 2.5 nm -25 xcexcm such as 2.5-250 nm or 250 nm-25 xcexcm, preferably within the intervals 25-250 nm or 250-700 nm. Also, the predetermined frequency range preferably comprises SPP frequencies corresponding to xcexSPP within the intervals 10 nm-100 xcexcm such as 10-1000 nm or 1-100 xcexcm, preferably within the intervals 100-1000 nm or 1000-3000 nm.
Often, it will be of interest that the predetermined frequency range comprises the frequencies of SPPs resulting from the coupling of photons having a wavelength xcexphoton in a predetermined wavelength range. The relationship between xcexSPP and xcexphoton depends on the material configuration of the device. Given a specific device, it is often possible to find a relationship between the wavelength xcexphoton of a photon and xcexSPP of the resulting SPP when the photon is coupled to the device. Hence, the predetermined frequency range preferably comprises SPP frequencies corresponding to the wavelengths xcexphoton of photons coupled to the device, said photons having a wavelength within the interval 100-20.000 nm.
Optionally, the predetermined frequency range comprises SPP frequencies corresponding to wavelengths xcexphoton within the interval 100-380 nm so as for the predetermined pattern to interact with SPPs resulting from the coupling of ultraviolet photons. Alternatively, the predetermined frequency range comprises SPP frequencies corresponding to wavelengths xcexphoton within the interval 380-780 nm so as for the predetermined pattern to interact with SPPs resulting from the coupling of visible photons. Preferably, the predetermined frequency range comprises SPP frequencies corresponding to wavelengths xcexphoton within the interval 780-20.000 nm so as for the predetermined pattern to interact with SPPs resulting from the coupling of infrared photons. Preferably, the predetermined frequency range comprises SPP frequencies corresponding to wavelengths xcexphoton within the interval 780-3.000 nm such as within the interval 1.100-2.000 nm in order for the resulting device to be used in the optical communication industry.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.